1. Field of the invention
The present invention relates to a thin film bulk acoustic wave resonator.
2. Related Art
The thin film bulk acoustic wave resonator utilizing the thickness longitudinal resonance of piezoelectric film is called a FBAR (Film Bulk Acoustic Resonator) or a BAW (Bulk Acoustic Wave) element, and has a good prospect on the application to RF filters and voltage controlled oscillators for mobile communication because dimension thereof is very small and a high excitation efficiency and sharp resonant characteristics can be obtained in a zone higher than GHz band.
In the thin film bulk acoustic wave resonator, resonant frequency thereof is determined by the acoustic velocity and the thickness of the piezoelectric film. Usually, the thickness of the piezoelectric film of 1 μm to 2 μm corresponds to 2 GHz, and the thickness of 0.4 μm to 0.8 μm corresponds to 5 GHz, and furthermore raising of the frequency up to several ten GHz is possible.
Performance of the thin film bulk acoustic wave resonator can be represented by the electromechanical coupling coefficient kt2 and the quality coefficient Q value.
The greater the electromechanical coupling coefficient kt2 becomes, the wider the bandwidth of the RF filter or the bandwidth of the voltage controlled resonator can be made. It is important that the resonator should be prepared by employing the piezoelectric crystal film having the high intrinsic electromechanical coupling coefficient kt2 and moreover aligning the polarization axis of the piezoelectric film crystal to the direction of the thickness of the film, in order to raise the electromechanical coupling coefficient kt2.
The quality coefficient Q value relates to insertion loss when the RF filter is formed, and to purity of the oscillation of the voltage controlled oscillator. Because the oscillation relates to various phenomena that will absorb elastic waves, a large value of the quality coefficient Q can be obtained by improving purity of the piezoelectric film crystal, or aligning the crystal orientation, or using the piezoelectric film with the aligned polarization direction.
Referring to FIGS. 10A to 1E, a structure and a fabricating method of a typical thin film bulk acoustic wave resonator of conventional art will be explained.
As shown in FIG. 10A, a recess 102 is formed on a silicon substrate 101 by anisotropic etching. At this moment, a naturally oxidized film 103 is formed on the silicon substrate.
Then, as shown in FIG. 10B, a sacrificial layer 105 (for example boron or phosphorus doped silicate glass, BPSG BoroPhosphoSilicate Glass), which is easy to be etched, is formed on the silicon substrate 101.
Thereafter, as shown in FIG. 10C, the surface of the sacrificial layer 105 is polished until the surface of the silicon substrate 101 appears at the portion other than the recess 102 of silicon.
Next, as shown in FIG. 10D, a lower electrode 111, a piezoelectric film 112 and an upper electrode 113 are deposited in order on the sacrificial layer 105. The layered structure 110 comprised of the lower electrode 111, the piezoelectric film 112 and the upper electrode 113 becomes a piezoelectric excitation portion.
Then, as shown in FIG. 10E, the lower electrode 111, the piezoelectric film 112 and the upper electrode 113 are drilled (not shown) up to the sacrificial layer 105, and then the sacrificial layer 105 is removed by selective etching, so as to expose the recess 102.
The piezoelectric resonator comes to completion by such processes. However, in this method, the sacrificial layer 105 must be removed after the layered structure of the lower electrode 111, the piezoelectric film 112 and the upper electrode 113, which is the piezoelectric excitation portion 110, is formed.
There is a problem that the piezoelectric excitation portion 110 is damaged because this etching usually requires long hours of treatment with strong acid or strong alkali, etc. Especially, following problems will occur, i.e. crystallinity of the piezoelectric film 112 is spoiled; the electromechanical coupling coefficient kt2 gets decreased; and the quality coefficient Q value deteriorates remarkably.
When the thin film bulk acoustic wave resonator is formed in an IC in which transistors, etc. have been fabricated in advance, there is a possibility of damaging the transistors, etc.
Besides, the thin film bulk acoustic wave resonator corresponding to multi bands, which has a plurality of thin film bulk acoustic wave resonators having different resonant frequencies formed on the same substrate, has been expected for the future mobile communication terminal.
For example, in the case of a cellular phone corresponding to multi bands, RF filters and voltage controlled oscillators corresponding to respective frequencies of band are necessary.
When an RF circuit using a channel filter is employed, RF filters with different frequencies in accordance with the channel number are required, even if the cellular phone is a single band type.
For example, because the band width of 60 MHz is divided into twelve channels of 5 MHz for W-CDMA (Wide-band Code Division Multiple Access) system, at least 12 kinds of narrow band thin film bulk acoustic wave resonators, each of which has a frequency different from that of the neighboring one by 5 MHz, are necessary.
For the structure of the conventional thin film bulk acoustic wave resonator explained in FIGS. 10A to 10E, it is necessary that the thin film bulk acoustic wave resonators with different thickness are formed on the same substrate by regulating the thickness of the piezoelectric film or the thickness of the electrode in order to correspond to each frequency. The reason is that the resonant frequency band can be changed by the thickness of the piezoelectric film or the thickness of the electrode.
However, to form the piezoelectric resonators, etc. with different thickness, number of lithography process for forming them in each thickness gets increased. This is a serious problem.
As mentioned above, the conventional thin film bulk acoustic wave resonator has a problem that the electromechanical coupling coefficient kt2 and the quality coefficient Q value are remarkably decreased because the piezoelectric film is damaged by etching process for forming the cavity.
In addition, the conventional thin film bulk acoustic wave resonator has another problem that number of lithography process for forming piezoelectric films of different thickness gets increased in order to correspond to the multi bands.
The present invention is intended to provide a thin film bulk acoustic wave resonator, which has a resonant excitation portion free from damage caused by etching, a high electromechanical coupling coefficient kt2 and a high quality coefficient Q value, in the light of the problems mentioned above.
The present invention is also intended to provide a thin film bulk acoustic wave resonator having a plurality of different resonant frequencies, which can be formed on the same substrate without increasing number of lithography process.